There has been proposed a diagnostic apparatus for conducting a test to track down a faulty region in a misfiring engine. See, Japanese Laid-Open Patent Publication No. 05-202801 (hereinafter referred to as “JP05-202801A”″). According to JP05-202801A, the diagnostic apparatus detects an air-fuel ratio in a given cylinder, approximately at a time when fuel stops being injected into the cylinder, and determines which one of a fuel injection system and an ignition system is responsible for a misfire in the cylinder based on the detected air-fuel ratio (Abstract). More specifically, according to JP05-202801A, if the fuel injection system is responsible for a misfire and fuel is not properly injected, then the air-fuel ratio remains essentially unchanged approximately at the time that fuel stops being injected into the cylinder. However, if the ignition system is responsible for a misfire and fuel is injected, then the air-fuel ratio changes approximately at the time that fuel stops being injected into the cylinder. Based on such a finding, the diagnostic apparatus can determine which one of the fuel injection system and the ignition system is suffering from a fault (see paragraph “0009”).